Citric acid esters as biogenic, regenerative fuels and heating materials

ABSTRACT

The invention describes citric acid esters as heating materials and fuels in stationary and mobile internal combustion engines and also in heating installations. Such propellants and fuels are characterized in particular by the fact that they burn completely without soot or with very little soot, therefore cause distinctly lower emissions of soot or fine dust than known fossil and regenerative fuels such as gasoline, diesel, kerosene, vegetable oils and biodiesel. Citric acid esters can be obtained completely from regenerative vegetable sources and are therefore CO 2  neutral.

The invention concerns citric acid esters and/or their derivatives as fuel and heating materials or as additive for those.

Fuel and heating materials for engines and heating installations are mainly hydrocarbons, which are obtained from crude oil of fossil origin. The finite nature of fossil resources is a disadvantage, a further is the accumulation of carbon dioxide related with the usage, which has been recognized as reason for the global warming. Soot and further products resulting from the incomplete combustion end up in the environment and damage the health of humans and affect the living conditions of the plants and animals.

Hence increasingly searching is carried out for biogenic, renewable resources for winning fuel and heating materials which are CO2 neutral or which improve the combustion and due to this reduce the discharge of damaging side and intermediate products of the combustion.

Known examples for this are plant oils, bio diesel produced therefrom (fatty acid methyl ester), bioethanol, and biobutanol.

It is an objective of the present invention to find new materials, hitherto not used as fuel or heating materials, which can be obtained from plants, bacteria, fungi or algae or which can be produced predominantly on the basis of vegetable raw materials.

This objective has been achieved in that the heating material or fuel completely or partially consists of citric acid esters or citric acid ester derivates. Citric acid triethyl ester or citric acid tributyl ester are preferred. Furthermore mixtures of various citric acid esters with each other are suitable.

Preferably known heating materials respectively fuels can be mixed with the citric acid esters or citric acid ester derivates with a ratio of

1-99%, preferably 5-50% and even more preferably 5-20%. In this way it has for instance been found that already at a mixture of 10% citric acid ester and 90% of commercially available diesel fuel as fuel for a diesel car considerably less, that is more favorable, exhaust values in view of the emissions of carbon monoxide and hydrocarbons can be achieved. In particular diesel, heating oil, kerosene, solid and liquid hydrocarbons, plant and animal oils and fats or fatty acid methyl esters are suited as conventional fuel components for such mixtures.

According to a further preferred embodiment the citric acid ester derivatives can comprise straight chains or branched alkyl or alkyl groups at the free alcohol group. Such citric acid ester derivatives can also be produced according to known methods by means of alkylation or acylation.

By means of the invention the following advantages are achieved: it has be found that the fuel respectively heating material according to the invention can combust in an engine or in a heating installation with a considerably reduced soot development than known fuels and heating materials from hydrocarbons, plant oils and biodiesel. The new fuel respectively heating materials thus can be classified as considerably cleaner than conventional fuels so that no soot filtering is required. Furthermore, they can be produced by means of conventional technologies using vegetable raw materials, for instance plants, bacteria, fungi or algae.

In the following the invention is described in more detail by means of examples. To begin with, the production of citric acid and its esterification is elucidated in principle, whereby reference is made to prior art. Citric acid today is being gained industrially by means of a transgenetic variant of the mold fungus Aspergillus niger. For this purpose there are mainly three conditions required:

-   -   1. high glucose and oxygen content in the culture medium     -   2. low pH-value (pH<3). This causes on the one hand that the         successor enzyme of the citricsynthetase in the citric cycle,         the aconitase, is being inhibited. Such a low pH-value is far         from the pH-optimum of the enzyme and due to this its activity         diminishes strongly. This leads to the situation that the citric         acid being formed is further metabolized only marginally by the         fungi. On the other hand the outer membrane of the fungi cells         gets instable and the citric acid is released into the outer         medium. In addition, the risk of contamination by undesired         extrinsic organisms is low at such low pH-values.     -   3. Low Fe²⁺-concentration (<5 mg/1). Due to this the aconitase         misses the cofactor. The Fe²⁺-ions are bound by the addition of         potassiumhexacyanidoferrat (III).

This method is for instance described in Rolf D. Schmid: Taschenatlas der Biotechnologie and Gentechnik (paperback atlas of the biotechnology and genetic engineering), 2. edition, Wiley-VCH publisher, Weinheim 2006, and further in Ulimanns Enzyklopadie der technischen Chemie (encyclopedia of the technical chemistry) 4. edition, Volume 9, publisher Chemie, Weinheim 1975.

Citric acid esters can be obtained by an acidic esterification of citric acid with the corresponding alcohols, for instance in accordance with the method according to WO 03008369 applicant DHW DEUTSCHE HYDRIERWERKE GMBH RODLEBEN (published 2003-01-30). Hereafter this methodology for the esterification of citric acid monohydrate with butanol is being elucidated.

In the esterification reactor 3.6 mol butanol is being provided and 1 mol citric acid monohydrate is dissolved therein. It is advantageous for this to employ butanol containing water (butanol content about 94 to 97%), which was provided in a preceding approach. The mixture is quickly brought to a temperature of about 100° C., at which the distillation of butanol-water-mixtures begins. The crystal water of the citric acid as well as the reaction water are together with the excess butanol released in vaporous form, condensed and led via a separator.

For supporting the water separation the reaction mixture is gasified with a small amount of nitrogen. In addition the inert gas atmosphere prevents the access of oxygen from air and the color change caused by this. The water-containing butanol (maximum 20% water, with advancing esterification a water fraction getting smaller) runs back into the reactor. During this first esterification step a) the reaction temperature increases to 125° C. After a reaction period of 5 hours the reaction mixture comprises an SZ<100 mg KOH/g, and the reaction velocity decreases considerably (recognizable by the reduction of the aqueous phase of the separator per unit of time).

At the beginning of the second esterification stage b) 0.5% (related to waterless citric acid) methane sulfon acid as about 20% solution is added to butanol. Due to this a considerable, but temporary increase of the reaction velocity is to be noted. The reaction temperature is continuously increased from 125° C. to 140° C. After 2 hours the second esterification stage is terminated with SZ<30 mg KOH/g.

At the beginning of the third esterification stage c) the butanol cycle via the separator is being prevented and waterless butanol is being dosed. The dosing velocity during the three hours lasting reaction period in the third esterification stage c) amounts to maximum 1 mol butanol/h for 1 mol of citric acid provided at a time. Due to this method of operation the remaining reaction water is quickly and almost quantifiably removed from the reaction mixture and a practically complete transformation of the citric acid with corresponding advantageous results in quality and yield of the end product is being achieved.

Examples for an almost soot-free combustion and for a practical application as diesel fuel are described in the following.

EXAMPLE 1

The combustion behavior of a citric acid triethyl ester produced in accordance with one of the above-described or other methods is being investigated by means of the following apparatus.

Citric acid ester is filled into a metal dish with diameter 12-15 cm with an edge of 3-5 cm until the filling height 1 cm below the upper edge of the edge is reached. A wick rolled from kitchen paper, which is stiffened and weighed down by means of a nail, is put in the middle of the metal dish. As soon as the wick has sucked up the citric acid ester, the wick is ignited. The citric acid ester is burning with a yellow flame. In doing so it is already clearly optically visible that no soot development is taking place.

EXAMPLE 2

In the same way as in example 1a citric acid tributyl ester produced in accordance with one of the above-described or other methods is being investigated. The same amount of citric acid tributyl ester in grams burns about 20% longer than the ethyl ester.

While burning the citric acid esters according to examples 1 and 2 a white porcelain dish is held inclined about 5 cm above the flame, so that the combustion products can hit against the porcelain. No or only very small traces of soot show up.

During comparative experiments with biodiesel, plant oil, diesel, petrol, kerosene and petroleum already the flame shows clear black soot formation. The porcelain dish is accordingly clearly covered black with soot.

APPLICATION EXAMPLES

As application example an engine test run has been carried out with tributyl citrate. The fuel consisted of a mixture of commercially available mineral diesel with tributyl citrate. Engine tests with this mixture have been carried out in an engine with the following specification

-   Type: FLPower 178 R air-cooled diesel engine of the Chinese company     Surgetho, engine displacement 296 ccm (0.296 l) year of construction     2006, power 5 PS. 7 test runs with the following mixtures of     tributyl citrate and diesel as well as without diesel additive.

Test run Mixing ratio/g/g Amount of diesel in Amount of tributyl No. Diesel/citric diesel kg citrate in kg 1 100/0  2 0 2 95/5  1.9 0.1 3 90/10 1.8 0.2 4 80/20 1.6 0.4 5 50/50 1 1 6 20/80 0.4 1.6 7  0/100 0 2

The mixture has be weighed on a scale, was put in a plastic container and has been intensively agitated for at least 10 minutes.

For the assessment of the soot formation in exhaust gases the exhaust gases have been led through a white paper towel (Tempo brand) for 5 minutes, which was span mounted in a distance of about 10 cm from the exhaust of the diesel engine.

The test has been carried out in each case 5 minutes after the start of a test run with a new mixture and 10 minutes prior to the end of the test run.

The residues of soot on the paper towel have been assessed in accordance to their color from black for pure soot to colorless with the following scale.

Assessment of the Soot Formation

-   -   0 white, only very low coloring     -   1 clear light coloring but no black soot     -   2 clear strong coloring but no black soot     -   3 coloring with some black soot     -   4 weak black coloring due to soot     -   5 dark black coloring due to soot

Run time of Result of the soot Test Standard diesel tributyl citrate the engine measurement *) run (%) (%) in h Beginning end 1 100 0 1 5 4 2 95 5 1 3 3 3 90 10 1 3 2.5 4 80 20 1 3 2.5 5 50 50 1 2 1.5 6 20 80 1 3 1 7 0 100 1 1 0 *) 2.5 means 2 to 3

For this see also FIG. 1.

The operation of the diesel engine in all test runs with inconspicuous. No difference between pure diesel and diesel with an increasing addition of tributyl citrate has been observed. A knocking of the engine as result of a worse combustion of the tributyl citrate has also not been determined. 

1. Heating material or fuel for a mobile or stationary combustion engine or heating installation with burner, characterized in that the heating material or fuel completely or partially consists of citric acid ester or citric acid ester derivates.
 2. Heating material or fuel according to claim 1, characterized by a citric acid triethyl ester.
 3. Heating material or fuel according to claim 1, characterized by a citric acid tributyl ester.
 4. Heating material or fuel according to claim 1, characterized by mixtures of citric acid esters with each other.
 5. Heating material or fuel according to claim 1, characterized in that known heating materials respectively fuels are added into the citric acid ester or citric acid ester derivates in a mixing ratio of 1-99%.
 6. Heating material or fuel according to claim 5, characterized in that the mixing ratio of citric acid ester or citric acid ester derivates and conventional heating materials respectively fuels is 5-50%, preferably 5-20%.
 7. Heating material or fuel according to claim 5, characterized in that the known heating materials/fuels consist of diesel, heating oil, kerosene, solid and liquid hydrocarbons, plant and animal oils and fats or fatty acid methyl ester.
 8. Heating material or fuel according to claim 1, characterized in that the citric acid ester derivates comprise straight chain or branched alky or alky groups at the free alcohol groups. 